Coal pulverizer associated with a rotary classifier and method for operating the same

Abstract: An improved coal pulverizer associated with a rotary classifier which can be operated always safely regardless of change of a species of coal, is disclosed. In a coal pulverizer associated with a rotary classifier which is applicable to the use of a pulverized coal burning boiler or the like, there are provided a function generator 1 having a signal C.F. representing a coal feed rate input thereto for outputting signals representing a preset proper range of a motor current of the coal pulverizer corresponding to the coal feed rate, a computing unit 2 responsive to a signal A representing the same motor current and signals representing a proper range of the motor current for outputting a command signal to maintain a number of revolution of the rotary classifier when the motor current represented by the motor current signal A is within the proper range while outputting a command signal to decrease or increase a number of revolution of the rotary classifier, respectively, in the case where the same motor current has increased beyond the proper range or in the case where the same motor current has decreased beyond the proper range, and a rotational speed controller 3 having the command signal input thereto for regulating a number of revolution of the rotary classifier.

Description: BACKGROUND OF THE INVENTION: 1. Field of the Invention: The present invention relates to a coal pulverizer provided with a rotary classifier applicable to a fuel feed system of a pulverized coal burning boiler or the like and to a method for operating the same. 2. Description of the Prior Art: In a heretofore known coal pulverizer associated with a rotary classifier applicable to a pulverized coal burning boiler, a method for controlling a rotational speed of a rotary classifier was such that either it was preset always at a constant number of rotation as a function of a coal feed rate as indicated by a curve a in Fig. 5 or it was preset to take a number of revolution represented by a number of segments as a function of a coal feed rate as indicated by a curve b . Under a same coal feed rate, if a rotational speed of the rotary classifier is increased, a grain size of pulverized coal at the outlet of the coal pulverizer becomes fine, hence a load factor of the coal pulverizer increases and a motor current also increases. On the contrary, if a rotational speed of the rotary classifier is decreased, a gain size of the pulverized coal at the outlet of the outlet of the coal pulverizer becomes coarse, and hence a load factor of the coal pulverizer as well as the motor current decrease. In the event that property (brand) of the coal fed to the above-described coal pulverizer has changed, for instance, in the case where it has changed from coal having high grindability (soft coal) to coal having low grindability (hard coal), a motor current of the coal pulverizer for the same coal feed rate increases. In the case of very hard coal, sometimes the motor current exceeded a rated current and resulted in motor trip. On the other hand, in the case where coal is very soft, in view of operation at a high efficiency of the boiler, it is desirable to operate at an as high as possible degree of pulverization, and to that end it is necessary to increase a motor current of the coal pulverizer by increasing a rotational speed of the rotary classifier. As a counter-measure against the above-mentioned problem, a method shown in Fig. 6 has been proposed, in which setting of a rotational speed of a rotary classifier is manually changed depending upon a brand of used coal. However, since coal is once fed to a large hopper called "coal bunker", it is difficult to precisely know when the brand of coal has changed. In addition, even with respect to a same brand of coal, it is not scarce that property (especially grindability) changes widely, and so, it was very difficult to properly set a rotational speed of the rotary classifier. A roller mill and its operation is described in JP 42 84 857. For the purpose of preventing noises and a roller mill from damage by an extraneous vibration without adversely affecting the grinding performance of the mill, the roller mill comprises means for detecting changes of torque fed into a grinding table rotated by a motor and at least one factor selected from among the number of rotation of a classifier, the angle of grinding blades and the load of a grinding roller which are all decided according to the so detected value. SUMMARY OF THE INVENTION: It is one object of the present invention to provide an improved method for operating a coal pulverizer associated with a rotary classifier, in which a current of a motor of the pulverizer can be prevented to exceed a rated value for avoiding a trip of the motor, while maintaining a high-efficiency operation of a boiler, and also to provide an improved coal pulverizer provided with a rotary classifier adapted to be operated according to the same method. According to the present invention there is provided a method for operating a coal pulverizer associated with a rotary classifier which is applicable to the use of a pulverized coal burning boiler or the like, wherein a rotational speed of said rotary classifier is controlled so that the motor current of said coal pulverizer falls within a preset range for any arbitrary coal feed rate to prevent the motor from tripping. According to the present invention, owing to the above-featured operation method of a coal pulverizer associated with a rotary classifier, the rotary classifier is operated at such rotational speed that a motor current of the coal pulverizer can be maintained within the preset range for any arbitrary coal feed rate. More particularly, for instance, in the event that coal being fed has changed to a hard species (brand) of coal at any arbitrary coal feed rate, the motor current rises and tends to exceed the preset range, but the rotational speed of the rotary classifier is controlled so as to decrease. Then, as the rotational speed of the rotary classifier decreases, a load factor of the coal pulverizer decreases, hence the rise of the motor current of the coal pulverizer stops (or the motor current decreases), and it falls within the preset range. In this way, according to the present invention, a motor current of a coal pulverizer associated with a rotary classifier is always maintained within a preset range, and so, the motor can be reliably operated without tripping. According to another feature of the present invention, there is provided a coal pulverizer provided with a rotary classifier, comprising a function generator responsive to a coal feed rate signal inputted thereto for outputting signals representing a proper range of a motor current of the aforementioned coal pulverizer corresponding to a given coal feed rate, a computing unit responsive to a motor current signal of the aforementioned coal pulverizer and the range signals of the motor current of the coal pulverizer output from the above-mentioned function generator for outputting a command signal to maintain a rotational speed of the above-mentioned rotary classifier when the motor current of the above-described coal pulverizer represented by the motor current signal is within the aforementioned proper range, while outputting a command signal to decrease or increase the rotational speed of the above-mentioned rotary classifier in the case where the above-described motor current has increased or decreased, respectively, beyond the above-mentioned proper range, and a rotational speed controller responsive to the command signal of the above-mentioned computing unit for regulating the rotational speed of the above-described rotary classifier. In the coal pulverizer having the above-featured construction, a coal feed rate signal is inputted to the function generator, and the function generator outputs signals representing a preset proper range of a motor current of the coal pulverizer corresponding to a given coal feed rate. A signal representing the motor current of the coal pulverizer and the signals representing a proper range of the motor current are inputted to the computing machine, and in the event that the motor current represented by the motor current signal has increased beyond the above-mentioned proper range, the computing unit outputs a command signal to decrease the rotational speed of the rotary classifier, while in the event that it has decreased beyond the above-mentioned proper range, the computing unit outputs a command signal to increase the rotational speed, and in the case where it is within the proper range, the computing unit outputs a command signal to maintain the same rotational speed. When the rotational speed controller has received the command signal from the computing unit, it regulates a rotational speed of the rotary classifier according to the command signal. In this way, the motor current of the coal pulverizer associated with a rotary classifier according to the present invention could be always maintained within a proper range even if the species of coal should change. Accordingly, the coal pulverized associated with a rotary classifier can be operated safely without tripping due to change of the species of coal. The above-mentioned and other objects, features and advantages of the present invention will become more apparent by reference to the following description of one preferred embodiment of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS: In the accompanying drawings: Fig. 1 is a block diagram showing a construction of one preferred embodiment of the present invention; Fig. 2 is a diagram for explaining an operation of the same preferred embodiment; Fig. 3 is a control flow chart of the same preferred embodiment; Fig. 4 is a diagram for explaining operations of the same preferred embodiment in the case where a species of coal being fed is changed during an operation (in contrast to similar operations of a coal pulverizer in the prior art); Fig. 5 is a diagram for explaining a operation of one example of a coal pulverizer associated with a rotary classifier in the prior art; and Fig. 6 is a diagram for explaining an operation of another example of a similar coal pulverizer in the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENT: Now one preferred embodiment of the present invention will be described with reference to Figs. 1 to 4. In Fig. 1, a coal feed rate signal C.F. representing a coal feed rate to a coal pulverizer associated with a rotary classifier is inputted to a function generator 1, and the function generator 1 outputs signals A 1 (upper limit) and A 2 (lower limit) representing a preset proper range of a motor current of the coal pulverizer corresponding to a given coal feed rate as shown in Fig. 2. An operating unit 2 receives a signal A representing a motor current of the coal pulverizer and the signals A 1 and A 2 issued from the function generator 1, compares these signals and outputs the following command signal: (a) If the motor current represented by the motor current signal A exists within the proper range represented by the proper range signals A 1 and A 2 , then a command signal ΔN = 0 to maintain the same rotational speed of the rotary classifier, is output. (b) If the motor current represented by the motor current signal A increases beyond the upper limit represented by the proper range signal A 1 , then a command signal ΔN < 0 to lower the rotational speed of the rotary classifier, is output. (c) If the motor current represented by the motor current signal A decreases beyond the lower limit represented by the proper range signal A 2 , then a command signal ΔN > 0 to raise the rotational speed of the rotary classifier, is output. For instance, as indicated by a point a in Fig. 2, if a motor current of the coal pulverizer represented by the motor current signal A is present within a proper range represented by the proper range signals A 1 (upper limit) and A 2 (lower limit), then a command signal ΔN = 0 to maintain the same rotational speed of the rotary classifier is output. In the event that during operation of the coal pulverizer, coal being fed has changed from coal having a high grindability to coal having a low grindability, a load factor of the pulverizer would increase, hence the motor current would increase, and the operating point in Fig. 2 would shift from the point a to a point b . However, when the operating point passes a point c on a straight line representing the proper range signal A 1 (upper limit), a command signal ΔN < 0 to lower the rotational speed of the rotary classifier, is output A rotational speed controller 3 receives the command signal ΔN issued from the computing unit 2 and controls the rotational speed of the rotary classifier according to the input command signal. The above-described mode of control is represented by a flow chart in Fig. 3. The effects and advantages of the above-described embodiment of the present invention is diagramatically disclosed in Fig. 4 as compared to the operations of a coal pulverized associated with a rotary classifier in the prior art. In this figure, the operations of the coal pulverizer according to the illustrated embodiment of the present invention are shown at (a), while the operations of the coal pulverizer in the prior art are shown at (b). As best seen from Fig. 4, when a coal feed rate is kept constant, in the case of the illustrated embodiment of the present invention, even if the coal being fed should change from soft coal to hard coal, the rotational speed of the rotary classifier is lowered, hence a load factor of the pulverizer is not increased so much, thus the motor current of the coal pulverizer can be maintained within the proper range, and therefore, the coal pulverizer can operate safely. On the other hand, in the case of the coal pulverizer associated with a rotary classifier in the prior art, since the rotational speed of the rotary classifier is constant, if the coal being fed should change from soft coal to hard coal, a load factor of the coal pulverizer is increased, hence the motor current of the coal pulverizer increases and eventually reaches a rated value, i.e., the upper limit value of the motor current, and therefore, tripping of the motor would occur. As described in detail above, according to the present invention, a rotational speed of a rotary classifier in a coal pulverizer is controlled in such manner that a motor current of the coal pulverized corresponding to a given coal feed rate can be maintained within a proper range by making use of a function generator, a computing unit and a rotational speed controller. Consequently, even if a coal feed rate and/or a species of coal should change, always the coal pulverizer could operate safely without generating a trip of the motor.

Claims: A method for operating a coal pulverizer associated with a rotary classifier which is applicable to the use of a pulverized coal burning boiler or the like, wherein a rotational speed of said rotary classifier is controlled so that the motor current of said coal pulverizer falls within a preset range for any arbitrary coal feed rate to prevent the motor from tripping. A coal pulverizer provided with a rotary classifier which is applicable to the use of a pulverized coal burning boiler or the like, comprising a function generator (1) responsive to a coal feed rate signal (C.F.) input thereto for outputting signals representing a proper range of a motor current of said coal pulverizer corresponding to a given feed rate; a computing unit (2) responsive to a motor current signal (A) of said coal pulverizer and the range signals (A 1 , A 2 ) of the motor current of the coal pulverizer output from said function generator (1), for outputting a command signal (ΔN) to maintain a rotational speed (N) of said rotary classifier when the motor current of said coal pulverizer represented by said motor current signal (A) is within the before mentioned proper range, while outputting a command signal (ΔN) to decrease or increase the rotational speed (N) of said rotary classifier in the case where said motor current has increased or decreased, respectively beyond said proper range; and a rotational speed controller (3) responsive to the command signal of said computing unit (2) for regulating the rotational speed (N) of said rotary classifier.

Inventors: IIDA YUTAKA, KINOSHITA MASAAKI, IIDA, YUTAKA, KINOSHITA, MASAAKI, Iida, Yutaka, c/o Nagasaki Ship. &amp; Mach. Works, Kinoshita, Masaaki, c/o Nagasaki Ship.&amp; Mach.Works
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1994
Main classification: B02C25
IPC codes: B02C25, B02C15, B07B7